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A320445 Primes p such that 2*p-1 and the concatenation of p and 2*p-1 are primes. 1
2, 367, 661, 691, 997, 1459, 2011, 2557, 2707, 3061, 3967, 4027, 4177, 4357, 4639, 5749, 6211, 6229, 6961, 7537, 7561, 7951, 8191, 8629, 8689, 9619, 10789, 10837, 11311, 12619, 13009, 13249, 13417, 13681, 14419, 14461, 14821, 15121, 15277, 15427, 15541, 15667, 15739, 15991, 16519, 17137, 17257 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Primes p arising in A320442.

All terms but the first == 1 (mod 6).

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

EXAMPLE

367 is in the sequence because 367, 2*367-1=733, and 367733 are all primes.

MAPLE

filter:= proc(p) local q, r;

   if not isprime(p) then return false fi;

   q:= 2*p-1;

   r:= p*10^(1+ilog10(q))+q;

   isprime(r) and isprime(q);

end proc:

select(filter, [2, seq(i, i=7..200000, 6)]);

PROG

(PARI) isok(p) = isprime(p) && isprime(2*p-1) && isprime(eval(concat(Str(p), Str(2*p-1)))); \\ Michel Marcus, Jan 10 2019

CROSSREFS

Cf. A320442.

Sequence in context: A013506 A013512 A295174 * A142532 A329555 A280281

Adjacent sequences:  A320442 A320443 A320444 * A320446 A320447 A320448

KEYWORD

nonn,base

AUTHOR

J. M. Bergot and Robert Israel, Jan 09 2019

STATUS

approved

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Last modified January 24 10:33 EST 2020. Contains 331193 sequences. (Running on oeis4.)